Techniques of changing a contact angle of water on an article surface have been known for long. In the field of coating, there are known, for example, a technique of forming a coating film having a low contact angle of water from a hydrophilic coating, a technique of forming a coating film having a high contact angle of water from a hydrophobic coating, a technique of applying a water- and oil-repellent agent and the like. Those techniques are intended to fix an article surface to either of hydrophilic or hydrophobic permanently if possible.
With respect to techniques of changing a contact angle of water on an article surface by a method other than the above-mentioned methods of forming a film, there are known (1) a technique of forming a hydrophilic functional group on an article surface by applying energy such as corona discharge or plasma (JP2000-109580A), (2) a technique of making an article surface hydrophilic by irradiation of light to a photocatalytic semiconductor material (for example, titanium oxide and the like) being capable of making a state of the surface hydrophilic by irradiation of light (Japanese Patent No. 2756474 and No. 2865065), (3) a technique of changing a contact angle of water by adjusting a temperature of heat-sensitive resin (T. Saitoh et al., Analytical Science 18, 203 (2003)) and the like. Those techniques (1) to (3) are also intended to make a hydrophobic article surface hydrophilic (to decrease a contact angle of water) or reversely to make a hydrophilic article surface hydrophobic (to increase a contact angle of water).
Also a technique of making an article surface hydrophilic and hydrophobic alternately by changing a contact angle of water by applying external energy (JP2001-158606A). This technique is for making an article surface hydrophilic (decreasing a contact angle of water) by irradiation of light to a photocatalytic semiconductor material (for example, titanium oxide and the like) and then making the article surface hydrophobic again by applying a mechanical shock (friction force) on the surface. This patent publication says that switching to hydrophilic and hydrophobic can be conducted at high speed.